JSP&Servlet学习笔记----第5章

Servlet进阶API

每个Servlet都必须由web容器读取Servlet设置信息（标注或者web.xml）、初始化。
对于每个Servlet的设置信息，web容器会为其生成一个ServletConfig作为代表对象，从中可以取得Servlet初始化参数，以及代表整个web应用
程序的ServletContext对象。
Web容器启动后，会读取Servlet设置信息，将Servlet类加载并实例化，并为每个Servlet设置信息产生一个ServletConfig对象，而后调用Servlet接口的
init()方法，并将产生的ServletConfig对象当做参数传入。
这个过程只会在创建Servlet实例后发生一次。
GenericServlet：
@Override
public void init(ServletConfig config) throws ServletException {
    this.config = config;
    this.init();
}
public void init() throws ServletException {
    // NOOP by default
}
ServletConfig相当于Servlet的设置信息代表对象。
可以使用标注设置Servlet初始参数
 */
@WebServlet(name = "FiveServlet",
        urlPatterns = {"/five.do"},
        initParams = {
            @WebInitParam(name = "name", value = "xiya"),
            @WebInitParam(name = "age", value = "24")
        })
private String name;
private String age;
@Override
public void init() throws ServletException {
    name = getInitParameter("name");
    age = getInitParameter("age");
}
或者web.xml
<servlet>
    <servlet-name>FiveServlet</servlet-name>
    <servlet-class>FiveServlet</servlet-class>
    <init-param>
        <param-name>name</param-name>
        <param-value>xiya</param-value>
    </init-param>
    <init-param>
        <param-name>age</param-name>
        <param-value>25</param-value>
    </init-param>
</servlet>
<servlet-mapping>
    <servlet-name>FiveServlet</servlet-name>
    <url-pattern>/five.do</url-pattern>
</servlet-mapping>
web.xml中的设置会覆盖标注的设置。（标注的name属性需要和web.xml的<servlet-name>匹配）。

ServletContext：作为整个Web应用程序的代表。

应用程序事件、监听器

Web容器管理Servlet/JSP相关的对象生命周期，我们可以使用监听器处理对HttpServletRequest对象、HttpSession对象，ServletContext对象的生成、销毁事件，实际上就是我们编写回调函数，然后由Web容器来调用。

HttpServletRequest事件、监听器

ServletRequestListener:是生命周期监听器。
ServletRequestAttributeListener是属性改变监听器。
public interface ServletRequestListener extends EventListener {

    /**
     * The request is about to go out of scope of the web application.
     * The default implementation is a NO-OP.
     * @param sre Information about the request
     */
    public default void requestDestroyed (ServletRequestEvent sre) {
    }

    /**
     * The request is about to come into scope of the web application.
     * The default implementation is a NO-OP.
     * @param sre Information about the request
     */
    public default void requestInitialized (ServletRequestEvent sre) {
    }
}
测试代码：
import javax.servlet.*;
import javax.servlet.annotation.WebListener;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

/**
 * Created by N3verL4nd on 2017/3/1.
 */
@WebListener
@WebServlet(name = "ListenerServlet", urlPatterns = {"/listener.do"})
public class ListenerServlet extends HttpServlet implements ServletRequestListener, ServletRequestAttributeListener{

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("doPost");
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("doGet");
        request.setAttribute("name", "xiya");
        request.setAttribute("name", "xiya1");
    }

    @Override
    public void requestDestroyed(ServletRequestEvent sre) {
        System.out.println("requestDestroyed");
    }

    @Override
    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("requestInitialized");
    }

    @Override
    public void attributeAdded(ServletRequestAttributeEvent srae) {
        System.out.println("attributeAdded");
        System.out.println("attributeAdded: " + srae.getName() + " " + srae.getValue());
    }

    @Override
    public void attributeRemoved(ServletRequestAttributeEvent srae) {
        System.out.println("attributeRemoved");
        System.out.println("attributeRemoved: " + srae.getName() + " " + srae.getValue());
    }

    @Override
    public void attributeReplaced(ServletRequestAttributeEvent srae) {
        System.out.println("attributeReplaced");
        System.out.println("attributeReplaced:" + srae.getName() + " " + srae.getValue());
    }
}
访问：http://localhost:8080/jspRun/listener.do
输出：
requestInitialized
attributeReplaced
attributeReplaced:org.apache.catalina.ASYNC_SUPPORTED true
doGet
attributeAdded
attributeAdded: name xiya
attributeReplaced
attributeReplaced:name xiya
requestDestroyed

HttpSession事件、监听器

public interface HttpSessionListener extends EventListener {

    /**
     * Notification that a session was created.
     * The default implementation is a NO-OP.
     *
     * @param se
     *            the notification event
     */
    public default void sessionCreated(HttpSessionEvent se) {
    }

    /**
     * Notification that a session is about to be invalidated.
     * The default implementation is a NO-OP.
     *
     * @param se
     *            the notification event
     */
    public default void sessionDestroyed(HttpSessionEvent se) {
    }
}

在HttpSession对象初始化或结束前，分别调用sessionCreated()与sessionDestroyd()方法。

ServletContext事件、监听器

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebListener;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

/**
 * Created by N3verL4nd on 2017/3/1.
 */
@WebListener
@WebServlet(name = "ContextListenerServlet", urlPatterns = {"/contextListener.do"})
public class ContextListenerServlet extends HttpServlet implements ServletContextListener{
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("doGet");
    }

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("contextInitialized");
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("contextDestroyed");
    }
}
[2017-03-01 04:02:31,096] Artifact jspRun:war exploded: Artifact is being deployed, please wait...
[2017-03-01 04:02:31,097] Artifact WeiBo:war exploded: Artifact is being deployed, please wait...
contextInitialized
[2017-03-01 04:02:31,448] Artifact jspRun:war exploded: Artifact is deployed successfully
[2017-03-01 04:02:31,450] Artifact jspRun:war exploded: Deploy took 354 milliseconds
由此可见，当整个Web应用程序加载Web容器之后，容器会生成一个ServletContext对象。
重新部署后的日子信息：
[2017-03-01 04:03:53,921] Artifact jspRun:war exploded: Artifact is being deployed, please wait...
[2017-03-01 04:03:53,922] Artifact WeiBo:war exploded: Artifact is being deployed, please wait...
contextDestroyed
contextInitialized
[2017-03-01 04:03:54,580] Artifact jspRun:war exploded: Artifact is deployed successfully
[2017-03-01 04:03:54,580] Artifact jspRun:war exploded: Deploy took 659 milliseconds
[2017-03-01 04:03:55,270] Artifact WeiBo:war exploded: Artifact is deployed successfully
[2017-03-01 04:03:55,270] Artifact WeiBo:war exploded: Deploy took 1,348 milliseconds

注：
生命周期监听器与属性改变监听器都必须使用@WebListener或在web.xml中设置，容器才会知道要加载、读取监听器的相关设置。

过滤器

它是介于Servlet与浏览器之间，可以拦截过滤浏览器对Servlet的请求，也可以改变Servlet对浏览器的响应。
在Servlet/JSP中要实现过滤器，必须实现Filter接口。并且需要使用@WebFilter标注或在web.xml中定义过滤器。
public interface Filter {
    public default void init(FilterConfig filterConfig) throws ServletException {}
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException;
    public default void destroy() {}
}
Filter接口的doFilter()方法类似于Servlet接口的service()方法。当请求来到容器，而容器发现调用Servlet的service()方法前，可以应用某过滤器，就会
调用该过滤器的doFilter()方法。
如果调用FilterChain的doFilter()方法，就会运行下一个过滤器，如果没有下一个过滤器，就会调用Servlet的service()方法。
如果没有调用FilterChain的doFilter()，则请求就不会继续交给接下来的过滤器或目标Servlet，这就是所谓的拦截请求。


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原文地址：https://www.cnblogs.com/lgh1992314/p/6616232.html